The Basics
The HackMaster GameMaster's Guide (GMG) is a 368-page paperback that has a suggested retail price of $29.99 and was written by the HackMaster Development Team: Jolly Blackburn, Brian Jelke, Steve Johansson and David Kenzer. The font is a bit small, but the small font certainly lets them pack in lots of material into those 368 pages, and boy, is it ever packed in. It has a useful Table of Contents and Index, and it's extremely easy to find what you're looking for. I don't care much about art in game products, so I won't mention much about it aside from the fact that the drawings by the Fraim brothers do an excellent job of defining the overall feel of the game, much like Tony DiTerlizzi's work did for Planescape.
What's Inside
The information in the HackMaster GMG gives you all that you need to inflict brutal critical hits on the characters in your game and to fully take advantage of all those annoying quirks and flaws that your players took during character creation to max out their stats and make them near-invincible. But there's more than that. Much more. It has the usual material that you would expect to find in a product aimed at a game master: all the details that you need to run encounters, the mechanics of advancing experience levels, running campaigns, lists of magic items, and so on. It's where you can find out the details on both the Hackmaster +12 and the Chain Mail Bikini of Remote Eye Gouging. It's also where you can learn about the Axe of Rapid Whacks and about the Boots of Groin Kicking. (OK, some of these aren't quite as serious as the others, but they're all in this book.) And since HackMaster is extremely compatible with AD&D, all of the material in this book can be used in your existing AD&D games.
The Bottom Line
If you're planning on running HackMaster, this book is essential; there's material in it that you absolutely have to have to run the game, but aside from this book and the Player's Handbook, but your AD&D books can be easily used for virtually everything else, so if you don't feel inclined to pick up all eight volumes of the Hacklopedia of Beasts (HackMaster's answer to the Monster Manual), you can easily do without them.
If you're not planning on running HackMaster there's still a good amount of material in this book that you can easily adapt to any game system, and if you're playing using either of the editions of AD&D then the material in this book will fit seamlessly into your game. Is this book an example of inspired genius that leaves you in awe of the creative powers of its authors? Perhaps not, but it's an extremely solid and useful book nevertheless, which should also appeal to players and DMs of the earlier (pre-d20) versions of the D&D game.